Various types of laser ignition devices have been considered. These laser ignition devices are used for internal combustion engines having low ignitability such as a gas fuel engine for cogeneration, a lean-burn engine or the like. Each of the laser ignition devices uses a semiconductor laser element as an excitation light source, irradiating a Q-switch laser resonator with excitation light oscillated by the semiconductor laser element to oscillate a pulse laser having high energy density. The energy density is further enhanced by using a condensing means that condenses the oscillated pulse laser in an air-fuel mixture introduced into the combustion chamber, so as to ignite the air-fuel mixture.
For example, PTL 1 discloses a laser ignition device performing ignition of an internal combustion engine, provided with a laser device including a passive Q-switch having a solid-state active laser. The solid state active laser includes a laser-active region and a laser-inactive region which are designed to have specific length.
On the other hand, in the semiconductor laser device, when light emitted from the light source of the semiconductor laser element or the like enters various optical elements, optical fibers or the like, a part of the light is reflected or scattered on an incident surface of the optical element or the optical fiber, and the light may partially return to the light source. In the case where the returning light enters an active layer of the semiconductor laser element, wavelength of the oscillation may be disturbed or the output thereof may be varied. In the worst case scenario, it has been known that the semiconductor laser element may be broken.
Therefore, according to a general semiconductor laser device, as a means for eliminating the returned light, an optical isolator is used. Specifically, a polarization dependent optical isolator which depends on a polarization state of the incident light, and a polarization independent optical isolator which does not depend on a polarization state of the incident light have been known (refer to PTLs 2 and 3).
Likewise, for the laser ignition device, by using the optical isolator, it is considered that the semiconductor laser element can be prevented from being damaged by the returned light.